Sequence Of Strong Earthquakes Research Articles

Stress Changes Modelled for the Sequence of Strong Earthquakes in the South Iceland Seismic Zone Since 1706

The South Iceland seismic zone is, roughly speaking, situated between two sections of the mid-Atlantic ridge, i.e., the Reykjanes Ridge southwest of Iceland and the Eastern Volcanic Zone on the island. It is a transform zone, where earthquakes are expected to occur on E-W-trending left-lateral shear faults, equivalent to conjugate, N-S-oriented right-lateral, rupture planes. In fact, earthquakes take place on en-echelon N-S-oriented faults, which is indicated by the distribution of main shock intensities, aftershocks as well as by surface fault traces. The stress field continuously generated in the fault zone by opening of the adjacent ridges is computed and superimposed on the stress field changes induced by a series of 13 earthquakes (M ≥ 6) between 1706 and 2000. The level of the pre-seismic stress field is analysed as well as the size of the area under high stress. Finally, the post-seismic stress field of June 2000 is analysed, to see where high stresses might have accumulated. The modelling indicates that the rupture planes located on separated parallel N-S-striking zones are dense enough to lead to an area-wide stress release by the series of events. The obtained pre-seismic stress level for most events is high and stable with the exception of situations when several strong shocks occur over a time span of several days, i.e., display typical main shock-aftershock patterns. The size of areas under high stress aside from of the rupture plane, i.e., where no event occurs at the specific time, is of medium to small size.

Changes in failure stress and stress transfer during the 1976-77 Friuli earthquake sequence

SUMMARY Coulomb stress changes related to the sequence of strong earthquakes that affected the Friuli area (Northeastern Italy) during the period 1976‐77 have been investigated to explore the possibility of fault interaction and stress transfer mechanisms. The seismic sequence that followed the Mw 6.4 mainshock of 1976 May 6, consists of 5 events of Mw between 5.2 and 6.0 during the same year and one event of Mw 5.3 that occurred in 1977 September, all characterized by thrust focal mechanisms. The area is affected by a regional compressive stress, which tends to activate N-dipping thrust planes. The goal of this study is to analyse and verify the relationship between the stress changes induced by the mainshock and the occurrence of subsequent events. Our results show that the mainshock induced a variation in the stress field in the area where most of the seismic events occurred. Our modelling shows a positive correlation between the areas of increased Coulomb stress induced by the mainshock and the distribution of seismicity that followed. This evidence suggests stress interaction and the possibility that the mainshock promoted the rupture of those faults associated with the event of 1976 May 9 (Mw 5.1) and the events of 1976 September 11 (Mw 5.2 and 5.6), 1976 September 15 (Mw 6.0) and 1977 September 16 (Mw 5.3). The Coulomb stress caused by co-seismic dislocations decreased in the zone where an Mw 5.9 earthquake occurred on September 15 at 03:15. When we included the contribution of the regional tectonic loading in the calculation of Coulomb stress changes, we found a positive correlation between the hypocentral area of the September 15 03:15 event and the area of Coulomb stress increase. Aftershocks that occurred after the mainshock but before the September 15 03:15 event are also located in the area of Coulomb stress increase.

Features of the Shear Wave Splitting and Stress Field in the Xinjiang‐Jiashi Strong Earthquake Region

AbstractIn recent years, much attention has been paid to the shear wave splitting in cracked medium. In this presentation we investigate the S‐wave splitting nearby the Jiashi strong earthquake sequence from the local event data recorded by the Jiashi portable seismic array. The fast polarization directions at each station nearby the Jiashi earthquake sequence have been obtained. The distribution of maximal compressive stress direction nearby the Jiashi earthquake sequence is inferred from the fast polarization direction of S‐waves. In our observations it has been found that on the side of the Tarim basin the fast polarization direction is toward the NS direction, which is consistent with previous results. In the fold deformation belt to the north of the Tarim basin, the maximal compressive stress gradually becomes in EW direction. Especially, nearby the Keping faults, the fast polarization direction becomes consistent with the strike of the fault plane of the Artushi earthquake. It is proposed from our results that the Jiashi earthquake sequence is caused probably due to the release of the localized tensile strain‐energy by lateral inhomogeneous deformation at the north edge of the Tarim basin.

Sequence of strong intraplate earthquakes in the Kodiak Island Region, Alaska in 1999–2001

A sequence of strong earthquakes was registered in 1999–2001 in the Kodiak Island region of the Alaska‐Aleutian subduction zone. Two Mw 7 earthquakes occurred in December, 1999 and January, 2001 and an Mw 6.5 event occurred in July, 2000. These events and their aftershocks recorded by the regional seismograph network were relocated using a Joint Hypocenter Determination (JHD) method. Regional broadband data were used to obtain seismic moment tensors for the main shocks and their largest aftershocks. Relocation and moment tensor inversion results indicate that these events originated inside the subducting Pacific plate. The focal mechanisms indicate down‐dip tension with the fault planes being nearly vertical and parallel to the strike direction of the subducting plate. The 1999 and 2000 events were located down‐dip of the locked portion of the megathrust, while the 2001 event was located directly beneath it.

Microseismicity and faulting geometry in the Gulf of Corinth (Greece)

SUMMARY During the summer of 1993, a network of seismological stations was installed over a period of 7 weeks around the eastern Gulf of Corinth where a sequence of strong earthquakes occurred during 1981. Seismicity lies between the Alepohori fault dipping north and the Kaparelli fault dipping south and is related to both of these antithetic faults. Focal mechanisms show normal faulting with the active fault plane dipping at about 45° for both faults. The aftershocks of the 1981 earthquake sequence recorded by King et al. (1985) were processed again and show similar results. In contrast, the observations collected near the western end of the Gulf of Corinth during an experiment conducted in 1991 (Rigo et al. 1996), and during the aftershock studies of the 1992 Galaxidi and the 1995 Aigion earthquakes (Hatzfeld et al. 1996; Bernard et al. 1997) show seismicity dipping at a very low angle (about 15°) northwards and normal faulting mechanisms with the active fault plane dipping northwards at about 30°. We suggest that the 8‐12 km deep seismicity in the west is probably related to the seismic‐aseismic transition and not to a possible almost horizontal active fault dipping north as previously proposed. The diVerence in the seismicity and focal mechanisms between east and west of the Gulf could be related to the diVerence in the recent extension rate between the western Gulf of Corinth and the eastern Gulf of Corinth, which rotated the faults dipping originally at 45° (as in the east of the Gulf ) to 30° (as in the west of the Gulf ).

Passive Seismic Experiment in the Xingjiang‐Jiashi Strong Earthquake Region and Discussions on its Seismic Genesis

AbstractFrom January 21 to April 16 of 1997, seven strong earthquakes took place in the Jiashi region of Xinjiang, China. This phenomenon had never happened in the Chinese continent, since earthquakes were recorded by instruments. To investigate the deep structural conditions of the Jiashi strong earthquake sequence, a portable broadband seismic array consisted of 30 stations was deployed in the Jiashi region of about 11,000 km2. This portable array crosses through the Tarim basin and the Tian Shan fold belt. The distance between stations is about 5–10 km. The frequency bandwidth of the observation system is 0.05–20 Hz.The 3‐D S‐wave velocity structure down to 100 km beneath the portable array was obtained by using the teleseismic receiver function migration and its nonlinear inversion techniques (Liu et al. 1996). From our observations the following conclusions are drawn: (1) The Moho depth on the side of the Tarim basin is about 40–52 km and it becomes 60–76 km on the side close to the Tian Shan. The crustal thickness beneath the fold belt nearby the south Tian Shan is much thicker than that of the Tarim basin and obvious structural deformation exists in the crust. (2) The north border of the Tarim basin is clearly laterally inhomogeneous. The contact deformation between Tarim and Tian Shan reveals the compressional movement in NNW direction. The topography of the Moho discontinuity beneath the array has an obvious correlation with the arctuate tectonics on the surface. The arctuate fold formation is closely related with the deep structure and the compression by the Tarim basin. (3) The Jiashi strong earthquake sequence mainly occurred above the gradient zone of the crust‐mantle boundary. The genesis of the Jiashi earthquake sequence is probably related to the activity of the conceal faults nearby the sources. This may be a result of the inhomogeneous deformation at the north border and the current tectonic movement of the Tarim basin. (4) The Jiashi strong earthquake sequence occurred mainly within the gradient zone from sediments to the crystalline crust beneath the fold belt with a lower shear module. This could be a reason why the stress drop of these events is much smaller than that of general cases. All of these demonstrate that the Jiashi earthquake sequence genesis is related not only to the current tectonic movement of the Tarim basin, but also to the feature of the deep structures. This should be studied further by numerical stress modeling.

Predictions of the 1997 strong earthquakes in Jiashi, Xinjiang, China

Abstract A sequence of strong earthquakes occurred in Jiashi County, Xinjiang, China (40°N, 77°E, population 280,000) in 1997, after a 10-month quiescence following a Ms 6.9 earthquake on 19 March 1996 in the same area. From 21 January 1997, through 18 October 1997, 15 earthquakes occurred with Ms ≥ 5.0, seven of which with Ms ≥ 6.0. Such sequences of strong earthquakes are rare around the world, but have occurred six times in the land area of China during this century. Of the 15 Ms ≥ 5.0 earthquakes in the Jiashi sequence, eight fell within the predicted 7-day time windows of a total of 8 short-term and imminent-stage predictions, but the initial shocks of the sequence (January 21, Ms 6.4, 6.3) were not predicted. Three of the eight predictions made were not fulfilled, one of which was announced to the public (a false alarm). Four of the remaining five predictions were acted on by local authorities and the public was notified 2.5 h to 4 days before the earthquakes, which caused no deaths. One earthquake (April 11, Ms 6.6) struck 30 min after the prediction was made but before action could be taken by the local authorities, and it caused nine deaths. These predictions were based on composite observations of precursory anomalies in seismicity, electromagnetic fields, and crustal deformation, which were in turn compiled over the past 30 years in a nationwide earthquake monitoring program. Examples of precursory anomalies include: (1) a clear seismicity pattern of “quiescence—intensification—main shock(s)” was observed for the April 6 earthquakes (Ms 6.3, 6.4), which was previously applied to the prediction of the well-known 1975 Haicheng earthquake in Laoning, China; (2) distinct decreases in b-values preceded the earthquakes on March 1 (Ms 6.0) and April 6 (Ms 6.3, 6.4); (3) anomalous variation of geomagnetic vertical component preceded the April 6 earthquakes (Ms 6.3, 6.4); (4) sudden decreases in borehole strain preceded 8 of the earthquakes.

Relationship between the earthquake sequences of Tangshan and Xingtai and the three dimensional velocity structure

In this paper, based on the results of tomographic image of Tangshan and Xingtai areas, the relations between the characteristics of the two strong earthquake sequences and their three-dimensional velocity structures are studied. The research results indicate that: (1) Mosaic distribution of low-velocity bodies and high-velocity bodies, especially the existence of high-velocity bodies with large size in crust are the common basis of development of the two earthquake sequences. (2) Scale, depth, and heterogeneity of high-velocity and low-velocity bodies are the important factors to effect the characteristic of earthquake sequences. (3) The depth of the high-velocity body in Tangshan area is less than that in Xingtai area, which is the principal reason why the dominant focal depth and the biggest focal depth of Tangshan earthquake sequence are less than Xingtai’s. (4) The depth of the high-velocity bodies in Ninghe area is more than that in Tangshan-Luanxian area, which lead to the biggest magnitude and epicentral intensity are lower. These results could be helpful for predicting the main shock of strong swarm-type earthquakes and later strong aftershocks.

Understanding the 13 May 1995 western Macedonia earthquake: A paleoseismological approach

Paleoseismological research by means of trenching in the area that was affected by the Kozani-Grevena strong ( M s = 6.6) earthquake sequence, revealed evidence for past reactivations of the same seismogenic fault. Five trenches were excavated along the Palaeochori-Sarakina part of the fault, in which three surface faulting paleoevents were identified at ca. 8.97, 36.7 and 72.5 ka BP (TL dates). Recurrence interval based on these datings is about 30 ka, which is very long, verifying the ‘low seismicity’ status of the area. On this basis, the 1995 earthquake was an out of sequence event, because the elapse time since the last major event is 8.97 ka instead of 30. Assuming a constant rate of strain accumulation, this would also explain the small amount of surface displacement that was observed during the 1995 earthquake (maximum 18 cm, usually up to 10 cm) in respect to the displacements observed in the trenches (> 25 cm) for previous paleoevents.

Mathematical simulation of moderate and strong earthquake sequence in Zhujiang Delta and its surrounding areas in this century and discussion on seismic risk regions

With the orthogonal design and the finite element methods, the outside stresses acting on the boundary and the inside tectonic stress field before the 1911 Honghai Bay earthquake are obtained. Under these stress fields, the dislocation patterns of the faults are consistent with the observed ones. Using the softening unstabilization model for elastoplastic media to simulate the process of the earthquake occurrence, 5 moderate and strong earthquakes in these areas in this century are simulated. The results show that the moderate or strong earthquake happened only at the sections of the faults whose fault safety degree is zero. According to the present distribution of the fault safety degree, the authors predict the seismic risk zones there.

Comparison and analysis of the stress field in the source region of Tangshan and Lancang earthquake sequences

The state of the stress fields in the source region is analysed systematically, on the basis of the focal mechanisms of 167 events withMs⩾4.0 of Tangshan sequence and 163 moderate-small events of Lancang sequence respectively. The result indicates that the directions of the stress field in the source region are generally stable and the variation is not obvious after the occurrence of strong shock. The dominant orientations of focal mechanisms of the aftershocks are consistent with that of the mainshock, there is still a small difference between some mechanisms of aftershocks and the mechanism of mainshock, and the difference decreases as time goes on. The numbers of solutions with dominant direction of Tangshan and Lancang sequences are similar to each other, it indicates that the controls of the strong earthquake sequences from the tectonic stress field are similar to each other too. Through the hierarchical clustering analysis of focal mechanism, it is found that the number of clusters of Tangshan sequence is larger than that of Lancang sequence, and their orientations are relatively disordered. It may be the cause that the tectonics in the Tangshan region are more complicated than those in the Lancang region.

Strong Alaskan earthquake sequence

Large earthquakes associated with Pacific‐North American plate interaction are common in southern Alaska. As the seismicity caused by this plate interaction diffuses into interior Alaska, earthquakes tend to become shallower, smaller, and less frequent.A recent sequence of large earthquakes in a remote area north of the Yukon River marks the farthest north that events of this magnitude have ever been recorded in the state. The epicentral region was centered about 10 km east of the trans‐Alaska oil pipeline and about 180 km northwest of Fairbanks (Figures 1 and 2). Parameters for the largest events recorded through March 16, 1985, are given in Table 1.,

Criteria of high seismicity, determined by pattern recognition

It is shown that in eastern Central Asia the epicenters of strong shocks with a magnitude ⩾6.5 are situated only in the disjunctive knots, i.e., in the zones of intersection of major faults active since the Neogene. A program for pattern recognition is applied to determine the knots where future epicenters of strong earthquakes may occur. Except for the information on the historical sequence of strong earthquakes in the region the data used are mainly a geomorphological description of the knots. The conclusions are confirmed by numerical experiments.